PWM to DC

Discussion in 'General Electronics Chat' started by ectmlk, Nov 4, 2008.

  1. ectmlk

    Thread Starter Member

    Feb 11, 2008
    21
    0
    Hey
    I need to convert a 20kHz pwm signal in to a DC reference voltage
    I have come across numerous examples of using a simple single-pole
    low pass filter. I have managed to simulate this in orcad using AC sweep,
    however when i go back to transient analysis and apply my Pulse the circuit makes no effort to average/filter the signal. I have also used the LM358 datasheet which shows it configured as a low pass but still doesn't work.
    Any ideas?

    Cheers
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit inverts the signal. When the signal goes negative then the output goes positive. When the signal goes positive then the opamp does nothing and stays near 0V.
    But the lousy old LM358 has trouble with frequencies over only 2kHz.
    Most better (but same price) opamps work fine up to 100kHz.
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    Greetings:

    A simple integrator is indeed the most straightforward means of decoding PWM. I would try an RC time constant equal to about twice the period of the waveform. See if that models properly. I suspect you might have had too long a time constant.

    eric
     
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